1. Field of the Invention
The present invention relates to a mixer and a converter including a mixer. More specifically, the present invention relates to a mixer used for converting frequencies from an RF signal to an IF signal and a converter including such a mixer in a communication apparatus or other apparatus utilizing a microwave band.
2. Description of the Related Art
In the past, in radio communication apparatuses, a mixer which mixes an RF (radio frequency) signal and an LO (local oscillation) signal has been used to generate an IF (intermediate frequency) signal that is easier to be processed in a circuit at the subsequent stage. As such a mixer, typically, a single balanced mixer or double balanced mixer which can efficiently reduce the leakage of an LO signal toward the output with a simplified circuit configuration is widely used.
FIG. 7 is a circuit diagram of a typical single balanced mixer. A single balanced mixer 50 includes an LO port 51, an RF port 52 for inputting an RF signal, an IF port 53 for outputting an IF signal, a balun 54 for converting an unbalanced signal to a balanced signal, and mixer diodes 55 and 56. The single balanced mixer 50 further includes a high-pass filter 57 for blocking LO and IF signals and for passing only an RF signal and a low-pass filter 58 for blocking LO and RF signals and for passing only an IF signal. An LO signal input from the LO port 51 is converted by the balun 54 into a balanced signal. The balanced signal is then applied to the node between the balun 54 and the mixer diode 55 and to the node between the balun 54 and the mixer diode 56, thereby driving the mixer diodes 55 and 56. An RF signal input from the RF port 52 passes through the high-pass filter 57 and is applied to the mixer diodes 55 and 56, while an IF signal generated by the mixer diodes 55 and 56 passes through the low-pass filter 58 and is output from the IF port 53.
In general, when such a single balanced mixer 50 is used for the microwave band, for example, a mixer (not shown) of the type in which the balun 54 includes microstrip lines in the same plane of a substrate is used.
According to the mixer 50 of the related art, while it is possible to provide a planarized circuit by forming the balun 54 with microstrip lines, there is a problem in that the balun takes up space and is not suitable for miniaturization because of its shape and size.